John Chrysostom

Then he adds, "When it is said, To-day if ye hear His voice, harden not your hearts, as in the provocation." This is a transposition, "when it is said, To-day if ye hear His voice, harden not your hearts." It must be read thus: "Let us fear lest a promise being left us of entering into His rest, any of you should seem to come short of it; for to us was the Gospel preached as well as unto them when it is said, To-day if ye hear His voice" (for "To-day" is "at every time"). …

Homily on Hebrews 6 ↗

Theophylact of Ohrid

He confirms in what sense he said "to the end," and says that this is signified by the expression "today." For "today" means: always, as he also said above: "but exhort one another every day, as long as it is called 'today'" (Heb. 3:13). Or, what is more approved, as stated by Saint John Chrysostom, the apostle makes a transition from the words "as long as it is said: 'today, when you hear His voice,'" to what follows: "Therefore let us fear, lest, while a promise remains" (Heb. 4:1), and so on.

Commentary on Hebrews ↗

Oecumenius

"As in the rebellion." Hyperbaton [ὑπερβατὸν] is: for we must understand it as "Let us fear." The surpassing thing; in what is said: Today, if you hear the voice, and the things following, let us fear, lest, a promise being left behind, and the things following.

The Pseudo-Oecumenian Catena on Hebrews ↗